Motion sensors that are capable of tracking movement of a finger or stylus over a surface are well known and widely used in systems, such as finger navigation systems, to move a cursor or input data. Generally, conventional motion sensors can be classified as one of two types depending on the means by which a surface of the finger or stylus is sensed, either optically or capacitively.
Optical navigation sensors use a laser or LED light source and photodiode array to detect motion based on either an image of the surface of the finger or a random intensity distribution of light known as speckle generated by scattering coherent light from a rough surface. Motion can then be determined by an algorithm using image correlation by detecting movement of the image of the surface of the finger, or by a spatial frequency calculation of the speckle. Optical navigation sensors typically have a high cost and a relatively large size due to a requirement for laser or light source, which must be internally shielded from the photodiode array, and the need for a precision optical assembly to project light reflected from the surface of the finger on to the photodiode array. Optical navigation sensors are also sensitive to ambient lighting, and can incorrectly detect motion when no surface is present due to changes in background light. Finally, image correlation motion calculation can be computationally intensive and requiring expensive processing circuitry.
The most common type of capacitive motion sensor is a capacitive trackpad, which uses a small array of capacitive sensors to detect a finger location, and determines motion by comparing a number of sequential finger locations. Thus, it will be understood that a significant disadvantage of capacitive trackpads is that they must always be larger than a surface of the finger, since a capacitive trackpad does not work if the surface to be tracked covers the entire array of capacitive sensors, and are generally much larger to detect the location of the finger in multiple sequential locations. Thus, a large trackpad is difficult to fit into many applications where surface area is limited. Additionally, a capacitive trackpad does not work for high speed motion due to time taken to detect location. Tracking higher-speed motion requires a larger array of capacitive sensors.
Another type of capacitive sensor is a fingerprint sensor. Fingerprint sensors use a capacitive array to detect a fingerprint image by sensing the capacitive differences between finger ridges and valleys. In the past there have been attempts to use fingerprint sensors as motion sensors by comparing subsequent images and performing an image correlation calculation at each possible offset to detect motion. However, speed is limited by the time required to capture the fingerprint image and/or the time to perform the image correlation. Faster motion requires faster image capture and faster calculations, limiting maximum speed, or increasing sensor area. Moreover, the calculations often require high-performance processors, increasing cost. Finally, the capture and transfer of fingerprint image gives rise to many security and privacy concerns.
Accordingly, there is a need for an inexpensive motion sensor capable of detect motion high speeds, with less required processing power, and a smaller sensing area. It is further desirable that the motion sensor achieves these objectives without giving rise to the security concerns that result from detecting and storing a fingerprint image.